Can you try adding a dependsOn configuration to the EventSourceMapping to make sure the function is created first. Or is the function already created and you just want to add something?
Are you on the latest beta, or master? We’ve shipped something recently that could potentially solve this problem, otherwise can you run serverless deploy --noDeploy so we can see the full CF template and maybe you can see an issue there.
What is the error Cloudformation is showing in the webUI (soon we will show those errors on the command line as well)?
Hmm this might be solved with a few recent merges we did, but there is also an issue with a race condition that happens sometimes with Roles. Can you try with the latest master?